Assigning faces to new materials
I would check the transparency blend mode settings of the material in the Properties Editor, Material tab:



You don't have these settings visible if you have Cycles selected as the render engine, but EEVEE is used to display stuff in the viewport, so just in case they are not there, just switch the render engine to EEVEE temporarily:



Also check face orientation: 

Red would mean that normals are inverted.